1. Field of the Invention
This invention relates to an image sensing device for converting light signals into electrical signals. More in particular, the present invention relates to such an image sensing device including a plurality of photo-sensors arranged in the form of an array to be used in a facsimile system for reading light information.
2. Description of the Prior Art
Heretofore, solid-state image sensing devices have been greatly used as a reader for converting light image information into electrical signals for further processing or for the ease of transmission to a remote place. Conventional solid-state image sensing devices have been manufactured with the use of charge coupled devices (CCD) or Metal-Oxide-Semiconductor (MOS) devices. In other words, a conventional solid state image sensor had a structure in which a plurality of photo-receiving elements or picture elements were formed in the form of an array on the surface of a semiconductor substrate, thereby each of the photo-receiving elements outputing electrical signals in response to the amount of light impinged thereon. The plurality of photo-receiving elements were customarily formed by CCD or MOS elements with application of the well-known semiconductor device manufacturing technology.
These conventional image sensors with CCD or MOS elements are advantageous in the case where photo-receiving elements of high integration are required in a small area. However, the smallness of these conventional image sensors is disadvantageous in certain applications where an original to be scanned by an image sensor is relatively large. This is particulary true for a facsimile system which sometimes requires to read an original having the size of DIN A4 or B4.
Thus, when use is made of these conventional image sensors, a separate optical system must be combined to compensate the smallness of the conventional sensors for practical use. Such a combination then requires complicated optical adjustments and it also requires a high-precision alignment between the image sensors and the optical system. Since an optical system including lenses is used to form a highly reduced image, it tends to create distortion in the peripheral portion of the image which, in turn, lowers the resolving power. Moreover, such an optical system requires a long optical path which hinders to make the whole system compact in size. For example, given that an original to be read has A4 size (210 mm) and a sensor for reading the original has the size of 20 mm with the reduction rate of 10 to 1, if use is made of a lens having the focal distance of 50 mm, then the optical distance from the original to the sensor results in the length of 600 mm, which necessarily makes the whole system bulky. It is also to be noted that the cost of an optical system including lenses cannot be significantly lowered, and wiring of CCD or MOS image sensors calls for high-level skills, which, in turn, can be a cause for pushing up the cost.
Another conventional approach was to construct a one-to-one image sensor which could read an original image as it is without magnification or reduction. Such a conventional one-to-one image sensor is diagrammatically illustrated in FIG. 1, and it comprises a plurality (e.g., 1,728) of photodiodes 2 formed in a single array on the surface of a substrate or base plate 1 and a plurality (e.g., 54) of custom-made IC chips 5, each of which includes a 32-bit shift register 3 and a plurality of MOS transistors 4 connected to respective outputs of the shift register 3.
In operation of such a one-to-one image sensor, a clock pulse is applied to the shift registers 3 to turn the MOS switches 4 on sequentially from one to another. When a particular MOS switch 4 is turned on, the photodiode 2 connected to the MOS switch 4 thus turned on is reverse-biased by a voltage source 6 through a resistor 7, thereby charges are accumulated at the PN junction of the photodiode 2 in question. Then the charge-accumulated photodiode 2 is exposed to a light image L to discharge in response to the received amount of light during a period while the clock pulse completes one scanning cycle. When the MOS switch 4 is turned on again by the clock pulse, the photodiode 2 is recharged to compensate the amount of discharge due to light exposure, whereby an output signal is taken from this recharging current. This mode of operation ensures high sensitivity and high speed operation and is often referred to as the "charge accumulation mode" in this field of technology.
It is true that the prior one-to-one image sensor shown in FIG. 1 is advantageous because it requires no additional optical system in reading a light image. However, it is not free from disadvantages because it structurally requires so many expensive IC chips 5. Furthermore, when mounting these IC chips 5 onto the substrate 1, wiring of the IC chips 5 to the photodiodes 2 and other peripheral circuits are quite complex and expensive, which could push up the manufacturing cost to the same level as that of CCD image sensors.